1. Field of the Invention
The present invention relates to a method of adjusting the hardness of a metallic material, and more particularly, to a method of adjusting the hardness of a metallic material such as a shaft member in which a portion that requires toughness (hereinafter referred to as a tough portion) such as a threaded portion and a portion that should be made hard (hereinafter referred to as a hard portion) such as a bearing portion are formed adjacent to each other.
2. Description of the Related Art
There are various types of machine parts made of a metallic material, such as a shaft member material made of steel or alloyed steel in which a tough portion and a hard portion are formed adjacent to each other. Such machine parts include a crankshaft for a chain saw. In the crankshaft for the chain saw, a threaded or key way portion requiring toughness is formed adjacent to a bearing portion requiring hardness. Conventionally, the hardness of the metallic material in which tough and hard portions are to be formed adjacent to each other is adjusted in the manner described below: first, the surface of a tough portion is copper plated, and a hard portion is then carburized. At this time, the tough portion is protected from being carburized by the copper plated thereon. Subsequently, the copper plated on the tough portion is removed.
Such a conventional hardness adjusting method requires troublesome and expensive processes such as copper plating and removal of the copper plated, and is uneconomical because it employs consumably materials such as copper. Furthermore, it is difficult to continuously process a metallic material automatically in that method.